1. Field of the Invention
The invention relates to a pressure gauge for a pneumatic tire, more particularly to a pressure gauge which has a relatively high reliability and a relatively long service life, and which can be installed at different locations on a pneumatic tire.
2. Description of the Related Art
U.S. Pat. No. 5,289,161 discloses a pressure gauge for a pneumatic tire of a vehicle. The pressure gauge includes a casing having a hollow adaptor to be attached to the pneumatic tire, an inlet, and a finger piece for opening an inflating valve of the pneumatic tire. A diaphragm is disposed inside the casing adjacent to the inlet and is movable, in response to pressure imposed on the inlet, between a first position in which the pressure is below a predetermined low pressure limit, and a second position in which the pressure is above a predetermined high pressure limit. A spring-loaded push rod is mounted movably in the casing, and is moved by the diaphragm. A signal-transmitting unit is provided inside the casing, and has first and second switch devices which are controlled by the movement of the push rod. The first switch device activates the signal-transmitting unit to transmit a first code when the diaphragm is in the first position, thereby indicating an under-inflated condition. The second switch device activates the signal-transmitting unit to transmit a second code when the diaphragm is in the second position, thereby indicating an over-inflated condition.
Although the aforementioned pressure gauge is capable of indicating an under-inflated or over-inflated condition of the pneumatic tire to which it is applied, the pressure gauge suffers from the following drawbacks:
1. It is difficult to align the locations of the first and second switch devices with the moving path of the push rod. In addition, the first and second switch devices cannot be adjusted to adjust the high and low pressure limits of the pressure gauge.
2. The reliability of the pressure gauge is relatively low, and the service life of the pressure gauge is relatively short. This is due primarily to the diaphragm, which easily experiences elastic fatigue after prolonged application of the pressure thereon, and to the first switch device, which is acted upon by the push rod for most of the time.
3. The pressure gauge is designed for installation only on the inflating valve of the pneumatic tire.